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The desired information about the camera tilts can be derived from either 
the slope or the shift of the horizon image: 
a) an absolute determination of the camera rotation about an axis parallel 
to the optical axis of the horizon camera can be obtained from the 
inclination of the horizon with respect to the line connecting the 
horizontal fiducial marks. 
b) the vertical shift of the registered horizon with respect to the 
horizontal fiducial marks allows a computation of the rotation about 
an axis perpendicular to the optical axis of the horizon camera. The 
shift has to be reduced for the influence of the flying height on the 
theoretical position of the horizon. 
In general, only the latter possibility is used (yielding more accurate 
results) and then mainly in such a way that only relative tilt 
differences between different camera stations are determined. 
In many cases, the true horizon will not be visible, due to haze and cloud- 
cover. In this case, a haze or cloud horizon is used instead. This, however, 
imposes certain problems due to the fact that different parts of the horizon 
are depicted on different horizon photographs, and thus cannot be compared 
directly. 
Furthermore, the absolute height of the depicted haze or cloud horizon might 
differ with the progress of the flight, which can also introduce errors in 
the tilt determination. 
The accuracy of horizon-data 
A standard deviation of tip and tilt of approximately 2° can be expected 
under favourable meteorological and topographical conditions. 
This error will increase to 4°~6 C or larger under less favourable conditions. 
Geometrical meaning of horizon data in photogrammetrie systems 
The horizon data represent independent determinations of the absolute tip 
and tilt of each individual aerial photograph. 
The relative tilt determinations have to be corrected for the influence of 
earth curvature and flying height. Furthermore, allowance has to be made for 
additional shift and slope corrections to the derived tilts when only a haze 
or cloud horizon has been depicted. This index error has to be determined 
with the aid of one or more absolutely oriented stereomodels. 
The data derived from the horizon photographs are related to the axis system 
of the individual survey photographs. They have to be transformed to the 
axis system valid for the strip triangulation, unless they are applied 
directly - (with the aid of cross levels) - to the individual frame holders.